In the Applicant's published Application WO 91/01028 it was explained that it was important for each coin sensor element to be in a known predetermined position relative to the path of coins through the testing device, for the two sensor elements of a pair, when they are co-operating in pairs, to be accurately in register with each other and also to be accurately at a predetermined distance away from each other across the coin path and also, in some circumstances, for the opposed walls which define the coin path to be exactly a predetermined distance away from each other.
For the purposes of this application, as in the above-mentioned application, the term "sensor element" will be used to refer to a component which is mounted as a unit, whether it is a complete sensor as for example a single coil operating independently, or only part of a complete sensor as for example one of a pair of series or parallel connected coils, one of a pair of transmitter/receiver coils, or one of a pair of transmitting/receiving optical devices.
The above-mentioned application also explained that the hinge joining the two main parts of a coin testing mechanism has normally had a major function in positioning the two parts relative to each other when they are closed, so that any dimensional variations occurring in the manufacture of the hinge or in its assembly, or any distortions in the hinge which might be caused during clumsy operation in use, have produced corresponding geometrical variations between one device and another, or in an individual device at different times, and this has caused variations in performance between one device and another and in the performance of an individual device at different times of its life.
The above-mentioned application described an embodiment (which to a large extent will be described herein so as to assist in a full understanding of the present invention) in which the final positioning of the two parts was influenced by the hinge and was determined entirely by location-defining features independently of the hinge. They determined the spacing between the parts at three positions spaced apart to form a triangle and, since there were only three such positions, no relative rocking of the two parts was possible. At two of the three positions, the parts made, at least ideally, point-to-point and point-to-line contact respectively so that the contact at these two positions would substantially prevent both translational and rotational movement of the two parts relative to each other in their own plane. At the third position, there was point-to-surface contact which had exclusively a spacing function. It was explained that the particular forms of these location-defining features enabled manufacturing tolerance to be accommodated with minimal resulting variation in the relative positions of the parts.
This was to a large extent successful, but it was found in practice that the location-defining features needed to be spaced quite widely apart so as to minimise the small extent to which the relative positions of the two parts could vary in the rotational sense in their own plane. It was then found that the pressure urging the two parts together within the triangle could cause undesired and unpredictable flexing of one of the parts thus partly nullifying the advantage achieved. Making the triangle smaller reduces this effect but permits an undesirable increase in the amount of rotational variation available.